piece of conductive material can function as an antenna, however
work best when they're designed to resonate at the frequency being
transmitted or received. For simple antennas this is mostly a
function of their length. These ideas can be tested with an
inexpensive handheld radio transmitter and a compact electric field
transmitters can interfere with other users. Always use
the following guidelines:
levels low and transmission times short.
allow students to play with the equipment.
the radio's receiver before transmitting a signal to
the transmission lines.
transmit if someone is using the frequency.
on this page is provided in good faith. However,
restrictions on transmitters will vary from country to
country and are subject to change
The demonstrations described below can easily be
used as quantitative lab experiments for high school physics
classes. It is especially appropriate for AP physics classes which
discuss electric and magnetic fields. When used as a lab experiment,
only one radio transmitter is needed which is controlled by the
teacher. Students would use the electric field strength meter
to measure relative field strength.
- A simple monopole antenna works best when its
length is sized for it to resonate. This is roughly equal to 1/4 of
the wavelength it transmits or receives. See "How Antennas Work".
The electromagnetic radiation emitted from such an antenna is
polarized. Due to
this polarity, the receiving antenna has to be oriented in the same
direction for best reception.
- Procedure Demo 1: Antenna Polarity
- Place the Deluxe Field Strength Meter in a
horizontal position with the antennas fully extended. Turn the meter
to its maximum sensitivity. From a location
across the room, hold a handheld radio in a horizontal position and
press the transmit key. Note the reading on the Compact Field
Strength Meter and adjust its sensitivity downward until the meter reads
about 10% of full scale. If the meter fails to register, slowly walk
forward until it reads about 10%. Keep the transmitter in a
horizontal position and measure the distance to the meter. Take several
meter readings along with measurements of distance as you walk
forward until the meter reads 100%.
Plot the relative field strength versus distance. This should be a
non-linear curve in which the field strength strongly declines
with increasing distance. The exact form is hard to predict since
there are many possible sources of distortion in the electric field,
not to mention possible reflections of the wave. It's best to plot this on an overhead
transparency or in some other form which can be seen by the entire
Turn the radio transmitter to a
and repeat the process described above. Compare the new readings with the old ones.
should be much lower.
This demonstration of antenna polarity can be done
with a low powered FRS handheld radio. These are very inexpensive and
are available from Walmart or RadioShack for as little as $15. The demonstration can also
be done with any of the radios mentioned in the "Finding a Radio Transmitter"
section of the article in the Standing Electrical Waves Demonstration.
- Procedure Demo 2: Optimum Antenna Length
- Note: this demo has to be done with a radio
frequency has a corresponding wavelength of roughly 2
meters. In the
United States, this would fit either the MURS
business band or the HAM 2meter band (see "Finding a Radio Transmitter"
for additional input). Otherwise, the antenna on the Deluxe Field
Strength Meter will be either too short or too long.
Place both the radio transmitter and
Deluxe Field Strength Meter in a horizontal position where the meter
reads 100% at full sensitivity. Reduce the meter sensitivity to
about 3/4 of its maximum scale reading when the antennas of the Deluxe
Field Strength Meter are fully extended. Slowly collapse the antenna
and take periodic meter readings as the antenna collapses. It's best to plot
this on an overhead transparency or in some other form which can be
seen by the entire class. The plot should show a relative maximum
around the 1/4 wavelength size. For the frequencies mentioned, this
would be a length of about 0.51 meters (20 inches).
Deluxe Field Strength Meter from MFJ Enterprises.com
||This is a low cost meter. Other
types are available which may work equally well.
||handheld radio transmitter
||MURS business band or the HAM
2meter band is the best. (See "Finding a Radio Transmitter")
- This project was supported by a
National Science Foundation
Experience for Teachers grant as part of Clemson University's
Undergraduate Research Experience in Wireless Communications.
Special thanks is due to Dr. Chalmers Butler of Clemson University
for his guidance and input on the preparation of this page.
For more information about wireless communication and the electromagnetic spectrum visit The Hidden World of the Electromagnetic Spectrum.
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